Clim. Past, 15, 105–119, 2019https://doi.org/10.5194/cp-15-105-2019© Author(s) 2019. This work is distributed underthe Creative Commons Attribution 4.0 License.

The 4.2 ka event, ENSO, and coral reef developmentLauren T. Toth1 and Richard B. Aronson2

1US Geological Survey, St. Petersburg Coastal and Marine Science Center, St. Petersburg, FL 33701, USA2Department of Ocean Engineering and Marine Sciences, Florida Institute of Technology, Melbourne, FL 32901, USA

Correspondence: Lauren T. Toth (ltoth@usgs.gov)

Received: 31 July 2018 – Discussion started: 31 August 2018Accepted: 13 December 2018 – Published: 16 January 2019

Abstract. Variability of sea-surface temperature related toshifts in the mode of the El Niño–Southern Oscillation(ENSO) has been implicated as a possible forcing mecha-nism for the global-scale changes in tropical and subtropicalprecipitation known as the 4.2 ka event. We review records ofcoral reef development and paleoceanography from the trop-ical eastern Pacific (TEP) to evaluate the potential impact ofthe 4.2 ka event on coral reefs. Our goal is to identify theregional climatic and oceanographic drivers of a 2500-yearshutdown of vertical reef accretion in the TEP after 4.2 ka.The 2500-year hiatus represents ∼ 40 % of the Holocene his-tory of reefs in the TEP and appears to have been tied to in-creased variability of ENSO. When ENSO variability abatedapproximately 1.7–1.6 ka, coral populations recovered andvertical accretion of reef framework resumed apace. There issome evidence that the 4.2 ka event suppressed coral growthand reef accretion elsewhere in the Pacific Ocean as well. Al-though the ultimate causality behind the global 4.2 ka eventremains elusive, correlations between shifts in ENSO vari-ability and the impacts of the 4.2 ka event suggest that ENSOcould have played a role in climatic changes at that time,at least in the tropical and subtropical Pacific. We outline aframework for testing hypotheses of where and under whatconditions ENSO may be expected to have impacted coralreef environments around 4.2 ka. Although most studies ofthe 4.2 ka event have focused on terrestrial environments, wesuggest that understanding the event in marine systems mayprove to be the key to deciphering its ultimate cause.

1 Introduction

The abrupt climatic shift at∼ 4200 years before present (BP;expressed as years before 1950), known as the 4.2 ka event, isnow recognized by many scientists to be the most significantclimatic perturbation of the middle to late Holocene (Walkeret al., 2012; ka stands for thousands of years ago). On 25June 2018, the International Union of Geological Sciencesofficially subdivided the Holocene epoch, with the 4.2 kaevent marking the start of the late Holocene, or the Megha-layan age. Although the signature of the 4.2 ka event is notas ubiquitous as the north Atlantic cooling episode knownas the 8.2 ka event, which is the demarcation between theearly and middle Holocene, it is now clear that the 4.2 kaevent had global climatic, ecological, and cultural impacts(Mayewski et al., 2004; Weis, 2016). In most locations, the4.2 ka event manifested as extreme and abrupt changes interrestrial hydroclimate (Staubwasser et al., 2003; Marchantand Hooghiemstra, 2004; Walker et al., 2012; Xiao et al.,2018), which drove environmental changes (Marchant andHooghiemstra, 2004; Booth et al., 2005) and cultural col-lapses (Weiss et al., 1993; Staubwasser et al., 2003; Weiss,2016; Wu et al., 2016) on a global scale. The ultimatecause(s) of the event and its impact and connection with otherregional- to global-scale climatic changes remains unknown(Walker et al., 2012).

Whereas shifts in the climate of high-latitude regions ingeneral, and the northern Atlantic in particular, have beenimplicated in the majority of climate events during the lateQuaternary (Dansgaard et al., 1993; Mayewski et al., 2004;Booth et al., 2005; Walker et al., 2012), there is little evi-dence for a high-latitude driver of the 4.2 ka event (Walkeret al., 2012). The event is only weakly recorded in Green-land ice-core records (Johnsen et al., 2001; Marchant and

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106 L. T. Toth and R. B. Aronson: The 4.2 ka event, ENSO, and coral reef development

Hooghiemstra, 2004; Mayewski et al., 2004; Rasmussen etal., 2006): major cooling in the northern Atlantic, and theassociated onset of Northern Hemisphere neoglaciation, pre-ceded the 4.2 ka event by at least 1000 years (Svendsen andMangerud, 1997; Mayewski et al., 2004; Walker et al., 2012;Marcott et al., 2013). The lack of a clear high-latitude forc-ing mechanism for the 4.2 ka event has led some researchersto suggest that there may have been a tropical driver of theclimatic changes around this time (Marchant and Hooghiem-stra, 2004).

The most significant source of climatic variability in thetropics is the El Niño–Southern Oscillation (ENSO). Al-though ENSO originates in the tropical Pacific, it causesglobal-scale thermal anomalies and changes in hydroclimate(McPhaden et al., 2006). The climatological manifestationsof individual El Niño and La Niña events can vary, but manyof the broadscale impacts of ENSO are similar to the global-scale changes inferred to have taken place during the 4.2 kaevent. For example, some of the most striking changes as-sociated with the 4.2 ka event were apparent shifts in thestrength of the Asian monsoon (Staubwasser et al., 2003;Wang et al., 2003). ENSO variability has been closely linkedwith the dynamics of the Asian monsoon throughout theHolocene (Liu et al., 2000). Whereas El Niño events aretypically associated with a weaker monsoon and resultantdrought conditions, the monsoon is stronger during La Niñaevents (Liu et al., 2000; Wang et al., 2003). The similar-ity in the climatic changes associated with ENSO and the4.2 ka event, coupled with the evidence for increasing ENSOvariability around 4.2 ka (Conroy et al., 2008; Koutavas andJoanides, 2012; Carré et al., 2014), has led a number of re-searchers to suggest that ENSO may have played a salientrole in the 4.2 ka event (e.g., Marchant and Hooghiemstra,2004; Booth et al., 2005; Walker et al., 2012; Li et al., 2018).It is not yet clear, however, whether changing ENSO vari-ability around 4.2 ka is likely to have been an ultimate driverof broadscale climatic changes around this time or whetherENSO was a proximal response to other climatic forcing.

Coral reefs are excellent models for elucidating the causesand consequences of climatic excursions such as the 4.2 kaevent. Reef-building (hermatypic) corals, which secrete arag-onitic skeletons that build the geologic frameworks of thereefs, are highly sensitive to extremes in temperature andlight. As a result, coral reefs are among the most vulnerableecosystems to modern anthropogenic climate change (e.g.,Hoegh-Guldberg, 1999; Hughes et al., 2018). Photosyntheticdinoflagellates, called zooxanthellae, which live endosym-biotically within the cnidarian hosts, provide the animalcomponent of the coral holobiont (the entire animal–plant–microbial symbiosis) with nearly all of its energetic needs(Muscatine and Porter, 1977). Persistently high temperatureand light levels, which are now occurring with increasingfrequency as a result of anthropogenically driven climatechange, disrupt the mutualistic relationship between the coralanimal and the zooxanthellae, leading to coral bleaching and

Figure 1. Photographs depicting the typical states of coral reefsin Pacific Panama (a) before, (b) during, and (c) after an El Niñoevent. Photograph (a) shows an example of a well developed coralreef dominated by branching Pocillopora corals. Photograph (b)shows a reef dominated by Pocillopora experiencing high levels ofcoral bleaching during the 2015–2016 El Niño event. Photograph(c) shows a large area of Pocillopora rubble that has persisted sincethe 1982–1983 El Niño event.

associated morbidity and mortality (Fig. 1; Brown, 1987;Glynn, 1991; Hoegh-Guldberg, 1999; Donner et al., 2005;Anthony, 2016; and many others). Cold-water conditions arealso inimical to coral growth and reef development (Gins-burg and Shinn, 1994; Precht and Aronson, 2004; Lirmanet al., 2011; Kemp et al., 2016; Toth et al., 2018). Coralsare vulnerable to a variety of other stresses and disturbances,including sedimentation, subaerial exposure, nutrient load-ing, hurricane damage, low-pH conditions, and low salinity(Kleypas et al., 1999; Aronson and Precht, 2001; Budde-meier et al., 2004; Goldberg, 2013), which can increase asa result of (or in tandem with) climatic shifts in one directionor another. Reef frameworks and the coral skeletons that con-stitute them preserve well in the fossil and subfossil record,providing a historical record of the response of coral reefs toclimate change (Jackson, 1992). Moreover, many of the per-turbations listed above leave geochemical and taphonomictraces that can be used as proxies to reconstruct changes inenvironmental conditions (e.g., Cobb et al., 2013; Toth et al.,2012, 2015a, b; Flannery et al., 2018).

Whereas the majority of the records of the 4.2 ka eventhave come from terrestrial environments, the contempo-rary impacts of ENSO are often felt most keenly in marineecosystems. Understanding whether and how marine ecosys-tems responded to climatic changes around 4.2 ka is, there-fore, critical to discovering the ultimate drivers of the 4.2 kaevent. Here, we explore the hypothesis that ENSO playeda role in the 4.2 ka event by reviewing paleoecological andpaleoceanographic records from marine environments in thetropical Pacific. We focus on the long-term collapse of coralreef development in the tropical eastern Pacific (TEP), whichappears to be related to changes in ENSO. We conclude thatthe relationship between ENSO and the 4.2 ka event warrantsfurther study and we outline a conceptual framework for fu-ture studies to investigate the linkages between these climatic

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phenomena using records from coral reef environments in thetropical Pacific.

2 Climate and coral reef development in the tropicaleastern Pacific (TEP)

2.1 Ecology of coral reefs in the TEP

Holocene reefs of the TEP are, for the most part, smalland poorly developed, as is generally the case for reefs onthe eastern margins of ocean basins (Darwin, 1862; Cortés,1993). Regional and local diversities of hard corals are lowin the TEP. Several living reefs, however, overlie well de-veloped Holocene frameworks that preserve millennial-scalerecords of regional and larger-scale climatic variability, andthe responses of coral assemblages to that variability.

The shallowest habitats of contemporary reefs off the Pa-cific coast of Panama, ranging from 1 to 5 m water depth,are characterized by fields of branching corals of the genusPocillopora (Fig. 1a; Glynn and Macintyre, 1977; Cortés,1997; Glynn and Ault, 2000). The stands of Pocillopora aredominated by what has been termed P. damicornis, but theyinclude several other species in the genus (many or mostof which are hybrids; see below). Massive corals, primarilyPavona spp., Porites spp., and Gardineroceris planulata, be-come more abundant with increasing depth below the zone ofessentially monogeneric occupation by Pocillopora (Glynn,1976; Glynn and Maté, 1997).

Richmond (1987, 1990) suggested that P. damicornis inthe TEP had adopted a life history strategy of exclusive, ornearly exclusive, asexual reproduction. In this scenario, thepopulations were seeded by sexually produced planula larvaefrom the central Pacific, which were transported by the NorthEquatorial Countercurrent and by eastward equatorial flowduring El Niño conditions (Glynn and Ault, 2000). Oceano-graphic modeling exercises, however, cast doubt on Rich-mond’s proposed teleconnection between the central andeastern Pacific, at least so far as Pocillopora is concerned(Wood et al., 2016; Romero-Torres et al., 2018). Further-more, genetic analysis by Combosch et al. (2008) demon-strated that the eastern Pacific populations of Pocillopora re-produce sexually. The genetics showed a difference in re-productive strategy in the genus Pocillopora between theeastern Pacific and the rest of its range in the Indo-Pacific:Pocillopora reproduce sexually by broadcast spawning in theeastern Pacific, whereas they brood internally fertilized orparthenogenetic eggs in the central and western Pacific, andin the Indian Ocean.

Broadcast spawning is correlated with interspecific hy-bridization among the species of Pocillopora in the TEP.Hybridization in the TEP is in turn associated with a radi-cally different functional ecology of (hybridized) P. damicor-nis in the eastern Pacific, including a far more robust skele-tal morphology, ecological dominance, and the construc-tion of reef frameworks predominantly composed of Pocillo-

pora branches. Within the eastern Pacific, limited gene flowamong populations of (hybridized) P. damicornis (henceforthsimply “Pocillopora”) raises the possibility of adaptation toregional-scale oceanographic gradients of upwelling (Com-bosch and Vollmer, 2011).

2.2 Response to ENSO events

Contemporary populations of corals on reefs of the TEP (seeFig. 6.1 in Toth et al., 2017, for a map of locations wherecoral reefs occur) highlight the responses of coral popula-tions to sequential perturbations by ENSO. The 1982–1983El Niño was one of the strongest to affect the eastern Pacificin recent history (Glynn, 1988a; Glynn et al., 2001). (How-ever, the impacts of this event were less severe elsewhere inthe tropical Pacific.) Persistently elevated water temperaturesfor 14 months resulted in severe regional-scale bleaching ofcorals (Fig. 1b) and other cnidarians bearing endosymbioticalgae (Wellington and Glynn, 2007). Coral mortality rangedfrom 50 % in Costa Rica to 97 % in the Galápagos Islandsand correlated with the degree of thermal stress the reefsexperienced in each location (Glynn, 1990). Mortality lev-els were somewhat lower off the Pacific coast of Panama.In the Gulf of Panama, where seasonal upwelling is strong,coral mortality was 85 %. In the adjacent Gulf of Chiriquí,where seasonal upwelling is weak or absent, mortality wassimilar, averaging 76 % (Glynn, 1990, 1991; Glynn and Col-gan, 1992; Wellington and Glynn, 2007). The similarity inthe level of mortality throughout Pacific Panama reflects thefact that El Niño suppressed seasonal upwelling in the Gulfof Panama in 1982–1983 so the level of warming was similarin both gulfs (Glynn et al., 2001).

Ecological observations prior to 1982 suggested that densestands of Pocillopora had persisted throughout the TEP fordecades at least. Pocillopora had in fact been the dominantbenthic component in shallow reef habitats prior to the 1982–1983 event, covering up to 90 % of the available substratumat 3–5 m depth (Fig. 1a). Pocillopora proved especially vul-nerable to bleaching-induced mortality and vast fields werekilled, although mortality was variable on multiple spatialscales (Glynn, 1990; Macintyre and Glynn, 1990). A collat-eral effect of Pocillopora mortality was the decline of sym-biotic crustaceans that defend the corals against attack bythe predatory crown-of-thorns starfish, Acanthaster planci.At Uva Island, in the Gulf of Chiriquí, Acanthaster were ableto crawl across freshly killed thickets of Pocillopora to attackand kill massive corals that had not bleached as severely anddid not harbor the crustaceans (Glynn, 1985, 1990). Acan-thaster did not then and does not now occur in the Gulf ofPanama (Glynn, 2004).

Algal turfs growing on the dead-coral surfaces led toenhanced populations of regular echinoids by increasingtheir food supplies. Higher densities of grazing sea urchinsgreatly increased bioerosion and suppressed coral recruit-ment, resulting in a switch from net vertical reef accretion

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108 L. T. Toth and R. B. Aronson: The 4.2 ka event, ENSO, and coral reef development

to net framework erosion (Glynn, 1988a, b, 1990; Glynnand Colgan 1992; Eakin, 1996; Reaka-Kudla et al., 1996).Centennial-scale increases in the frequency of severe ENSOevents such as the one in 1982–1983 could have been respon-sible for the poor development of reefs observed in many ar-eas of the eastern Pacific (Glynn and Colgan, 1992; Glynn,2000).

A second extreme ENSO event killed an estimated 16 %of corals worldwide in 1997–1998 (Wilkinson, 2000). The1997–1998 and 1982–1983 events were of approximatelyequal magnitude and duration in the eastern Pacific. Bothevents may have been enhanced by global warming and theywere the two most intense events in the preceding 50 years(Hansen et al., 1999; Karl et al., 2000; Enfield, 2001; Hugheset al., 2018).

Coral mortality in the eastern Pacific was lower in 1997–1998 (Guzmán and Cortés, 2001; Vargas-Ángel et al., 2001).There was essentially no mortality in the Gulf of Panamabecause, whereas seasonal upwelling was suppressed duringthe 1982–1983 El Niño event, upwelling was unaltered in1997–1998 and cooled the water column (Riegl and Piller,2003; Glynn et al., 2001). Muted levels of coral mortal-ity elsewhere, including only 13 % mortality in the Gulf ofChiriquí, were correlated with the presence of thermally re-sistant zooxanthellae in the corals (Glynn et al., 2001; Bakeret al., 2004; see also D’Croz and Maté, 2004). Collateral ef-fects following the 1982–1983 event, particularly enhancedpredation by Acanthaster (in the Gulf of Chiriquí) and in-creased bioerosion by sea urchins, were also far less severeafter the 1997–1998 ENSO, due to precipitous declines ofboth taxa (Eakin, 2001; Glynn et al., 2001; see also Fongand Glynn, 2001). The effects of the 2015–2016 El Niñoevent were also minimal in Pacific Panama. Upwelling againbuffered the reefs in the Gulf of Panama from elevated tem-peratures, and most reefs in the Gulf of Chiriquí experiencedcoral bleaching, but only minor bleaching-related mortality.

Coral populations have recovered to varying degrees fromthe El Niño events in 1982–1983 and 1997–1998, apparentlyhaving been seeded by populations that persisted within refu-gia in the eastern Pacific (Wood et al., 2016). Coral coverdeclined in the Gulf of Chiriquí at 3–7 m depth after 1983.By 2002, coral cover was 12 % at Uva Island (down sig-nificantly from 35 %) and 8 % at Secas Island (down non-significantly from 11 %; Wellington and Glynn, 2007). Coralcover at Saboga Island in the Gulf of Panama recovered at2–3 m depth from 0 % in 1984 to the pre-ENSO level of50 % by 1992. At 3–5 m depth, some areas of the Sabogareef recovered to 50 % within a few years, but large areasof coral rubble have persisted to this day (Fig. 1c), and thishas also been the case on many other reefs. The areas of coralrubble consist primarily of taphonomically degraded Pocillo-pora branches, which are covered in algal turfs, encrusted bycoralline algae, and colonized at low frequency by the earlysuccessional coral Psammocora stellata.

In summary, elevated water temperatures and high irradi-ance (resulting from low cloud cover) associated with thestrong El Niño events in 1982–1983 and 1997–1998 in Pa-cific Panama caused widespread coral bleaching, which wasassociated with mass coral mortality in the earlier event(Glynn et al., 2001). La Niña is also problematic for coralsin Pacific Panama. Lowered sea level in the TEP during LaNiña events causes more frequent coral mortality associatedwith subaerial exposure (Eakin and Glynn, 1996; Toth etal., 2017). In addition, La Niña is associated with elevatedrainfall and enhanced upwelling in Pacific Panama. Elevatedrainfall increases turbidity, and enhanced upwelling reduceswater temperatures, decreases pH, and increases nutrient lev-els. All of these changes act to suppress coral growth (Glynn,1976).

Food webs on eastern Pacific reefs are complex despitetheir low diversity (Glynn, 2004). Corallivores other thanAcanthaster, including pufferfish (Tetraodontidae) and gas-tropods (Pediculariidae and Muricidae), have effects thatvary on multiple scales. On the other hand, because it is situ-ated close to the Equator, Panama has not experienced a hur-ricane since at least as early as 1871 (Neumann et al., 1999),removing hurricanes as a factor – but not storms and waveaction in the winter dry season.

2.3 The question of prior occurrence

The TEP has been affected by more than 30 strong ENSOevents during the past 500 years and perhaps hundreds ofevents as strong as the 1982–1983 and 1997–1998 eventsduring the Holocene (Colgan, 1990; Moy et al., 2002; Reinet al., 2005). Cores drilled from massive coral colonies (coralheads) throughout the Indo-Pacific display a trend towardmore intense ENSO events after 1976 (Urban et al., 2000);however, although ENSO variability during the last centurymay be significantly higher than many times in the past, thereis precedent in the fossil record for the intensity of recentevents (Cobb et al., 2013). Whereas gross patterns of reef de-velopment have been examined in the eastern Pacific (Glynnand Macintyre, 1977; Macintyre et al., 1992; Cortés et al.,1994), biotic turnover, its climatic drivers, and its geologicalconsequences remain understudied.

At least some effects of the 1982–1983 ENSO event wereunprecedented on a scale of centuries. First, colonies of themassive corals Porites lobata and Pavona clavus in the Galá-pagos were 350–425 years old at the time of their death dur-ing the event (Glynn, 1990). Second, Dunbar et al. (1994)examined a colony of Pavona clavus that was part of the reefcommunity at Urvina Bay in the Galápagos. In 1954, UrvinaBay was tectonically uplifted more than 7 m, preserving inexcellent detail the effects of a strong El Niño in 1941 (Col-gan, 1990). The Pavona colony, which had survived the 1941event, showed more than 350 years of continuous growthbefore its death in the 1954 uplift. Clearly, some massivecorals had lived through previous, strong ENSO events. On

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the other hand, partial colony mortality and regeneration ofthe massive species G. planulata at Uva Island in the Gulfof Chiriquí marked both the 1982–1983 and 1997–1998 EN-SOs, so even the latter event was not without ecological con-sequences (Wellington and Glynn, 2007).

More germane are Colgan’s (1990) observations on up-lifted stands of Pocillopora at Urvina Bay. Much of the Pocil-lopora was in poor taphonomic condition (but excellentlypreserved in that poor condition; Colgan and Malmquist,1987), undoubtedly the result of 13 years of bioerosion byurchin grazing after the ENSO of 1941 and prior to the uplift.The uplifted reef contained clear evidence of bioerosive un-dercutting and collapse of the Pocillopora framework causedby sea urchins, Eucidaris galapagensis, quite similar to whatwas observed in living communities in the Galápagos afterthe 1982–1983 event. Furthermore, Eucidaris tests were scat-tered in large numbers at the base of the eroded Pocilloporaframework.

The question of prior occurrence is important because, asdiscussed above, repeated El Niño events could explain whyreefs are poorly developed in the eastern Pacific. Push coresthat we extracted from uncemented reef frameworks in boththe Gulf of Panama and the Gulf of Chiriquí appear to cor-roborate the pre-1982 ecological data, suggesting continu-ous dominance of Pocillopora for at least the last 1500 years(Toth et al., 2012, 2017). The ecological and paleobiologi-cal lines of evidence are not strictly comparable, however,because the temporal precision of cores may not be greatenough to detect decadal-scale events. Can we identify masscoral kills and excursions from the Pocillopora-dominatedstate in the subfossil record of eastern Pacific reefs? The re-sults of Colgan (1990) and post-1983 observations through-out the region imply that under the right conditions it shouldbe possible to detect bioeroded horizons of Pocillopora andshifts in dominance between coral species.

Pocillopora-dominated reefs are not well developed in theeastern Pacific due to the narrow shelves on which they grow;exposure, in at least some locations, to cold and acidic up-welling waters; and episodic bleaching during ENSO events(Cortés et al., 1994; Glynn and Maté, 1997; Kleypas et al.,2006). Glynn and Colgan (1992) and Glynn (2000) arguedthat eastern Pacific reefs accrete poorly because the coralsare periodically killed by severe El Niño events and then bio-eroded by echinoids. Our study of the geologic record, incontrast, points to a protracted phase shift, during which thevertical accretion of Holocene reef frameworks off the Pa-cific coast of Panama was suppressed for far longer than thereturn time of strong ENSO events.

Our push cores from three fringing Panamanian reefs,which spanned a gradient of upwelling and water depths(−0.8 to −4.1 m relative to mean sea level), and whichwe dated with radiocarbon and uranium-series techniques,showed that vertical accretion essentially ceased from ∼4100 to 1600 BP (Fig. 2; Toth et al., 2012). There is no evi-dence that the reefs shifted to lateral accretion at this time, as

the reefs at all coring sites were growing over a paleodepthrange of ∼ 1.6 m at the time they shut down (Lauren T. Toth,unpublished data). The hiatus in growth lasted approximately2500 years, meaning the reefs were in a phase of negligi-ble growth for as much as 40 % of their history (Toth et al.,2012). Detailed core logs are provided in Toth, 2013).

Active coral growth was represented in the cores aslong intervals of well preserved Pocillopora branch frag-ments. The hiatus manifested as a thin layer of taphonom-ically degraded Pocillopora branch fragments, coralline al-gae, and Psammocora stellata, all of which characterizeENSO-generated rubble fields on contemporary reefs in Pa-cific Panama. During the rest of their history, however, thereefs grew upward at rates comparable with the vertical-accretion rates of Caribbean reefs (Toth et al., 2012). The“poor” Holocene development of these reefs is, therefore, aconsequence of the hiatus. The tempo and causality of reefdevelopment in Panama represent a vastly scaled-up versionof the earlier model of Glynn and Colgan (1992, 2000), whohad suggested that poor reef development was a consequenceof decadal-scale disturbance by El Niño. Although ENSOsuppresses populations of Pocillopora on subdecadal to mul-tidecadal scales in Panama, slowed growth on those scaleshas not been the cause of low bulk-accretion rates throughthe Holocene.

3 ENSO and coral reef collapse at 4.2 ka

There is now broad consensus across both climate modelsand paleoclimate reconstructions that the middle Holocenewas a period of relatively low ENSO variability (Sandweisset al., 2001; Tudhope et al., 2001; Rein et al., 2005; Koutavaset al., 2006; Zheng et al., 2008; Cobb et al., 2013; Carré etal., 2014; Liu et al., 2014; Emile-Geay et al., 2016; Leonardet al., 2016b; Thompson et al., 2017). In particular, the inter-val 5–3 ka stands out as a period when paleoclimate recordsfrom throughout the tropical Pacific show a significant reduc-tion in ENSO activity (Emile-Geay et al., 2016). Althoughthe exact timing of the middle-Holocene ENSO minimumvaries among reconstructions, some studies have suggestedthat there was exceptionally low ENSO variability between4.3 and 4.2 ka (Cobb et al., 2013; McGregor et al., 2013;Leonard et al., 2016b; Emile-Geay et al., 2016). Paradoxi-cally, ∼ 4.2 ka is also a point of inflection in most ENSOrecords, after which time ENSO variability increased, even-tually leading to the establishment of the modern ENSOregime (Conroy et al., 2008; Koutavas and Joanides, 2012;Carré et al., 2014).

ENSO appears to be a primary control on modern coralgrowth and reef development in the TEP (Glynn and Col-gan, 1992; Glynn et al., 2001; Toth et al., 2012, 2017). Thedominant role of ENSO in modulating the ecology of con-temporary reefs in the TEP led us to hypothesize that ENSOvariability may also have shaped the dynamics of Panama-

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110 L. T. Toth and R. B. Aronson: The 4.2 ka event, ENSO, and coral reef development

Figure 2. Coral-based reconstructions of oceanographic and climatic variability in Pacific Panama from 5500 BP to present (after Toth et al.,2015a, b). Plot (a) provides a reconstruction of upwelling intensity in the Gulf of Panama based on the location-specific radiocarbon-reservoirage offset, 1R (Toth et al., 2015b). Plots (b) and (c) provide reconstructions of temperature and salinity variability for Pacific Panama basedon 200-year running means (±95 % CIs, confidence intervals) of Sr /Ca and δ18O, respectively, of corals sampled from a core collected fromthe reef at Contadora Island in the Gulf of Panama. The individual data points used to generate the curves can be found in Toth (2013) andToth et al. (2015a). Modern values are means (±95 % CIs) measured from living corals collected from the same reef in 2011 (described inToth et al., 2015a). The timing of the 4.2 ka event and the hiatus in the development of reefs in Pacific Panama are indicated by the red andgray bars, respectively.

nian reefs over millennial timescales. We tested the hypoth-esis that changes in ENSO activity around 4.2 ka could havetriggered reef shutdown in Pacific Panama by evaluatinggeochemical proxy records from corals in our cores in theGulf of Panama (Fig. 2; Toth et al., 2015a, b). For obviousreasons, the availability of coral samples from the time ofthe hiatus was limited; however, we were able to analyzethe Sr /Ca and δ18O of seven coral skeletons that our agemodel suggested grew at the beginning of the hiatus (∼ 3.9–3.6 ka; Toth et al., 2015a). We also have a measurementof oceanic radiocarbon, a proxy for ocean circulation, fromone coral that grew during this period (Toth et al., 2015b).Therefore, although we do not have any records at precisely4.2 ka, our reconstructions give insights into the mean cli-matic and oceanographic states bounding 4.2 ka, which weevaluate in the context of records of ENSO variability fromthroughout the tropical Pacific presented in Fig. 3. Note thatthe schematic (Fig. 3) provides an overview of changes inENSO, suggested by our work and the paleoclimate litera-ture, that could have contributed to the shutdown of eastern

Pacific reef development. It is not meant as a comprehensivereview of the literature.

Our record of oceanic radiocarbon variability suggests thatupwelling was enhanced beginning at 4.3 ka and continuedto intensify over at least the next 500 years (Fig. 2a; Toth etal., 2015b). The hiatus in reef development was more pro-tracted in the Gulf of Panama, implying that upwelling mayhave made reefs more sensitive to further environmental per-turbations (Toth et al., 2012). Paleoclimate reconstructions,which we based on Sr /Ca and δ18O in the coral skeletons,suggest that these oceanographic changes were followed bya transition to a cooler, wetter climate beginning by ∼ 3.9 ka(Fig. 2b, c; Toth et al., 2015a). A lake record from the Galá-pagos recorded a contemporaneous period of anomalous dry-ing, which in that location points to enhanced La Niña activ-ity (Fig. 3; Conroy et al., 2008). Together, the inferred condi-tions indicate that an enhanced La Niña-like climate regimewas established just after 4.2 ka, which could have providedthe initial trigger that shut down reef development in PacificPanama for the next 2500 years (Toth et al., 2012, 2015a).

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Figure 3. Schematic diagram of the reconstructed changes inENSO variability in the tropical Pacific in relation to vertical reefaccretion in Pacific Panama. Colored bars are based on a paleo-ceanographic reconstruction from Pacific Panama shown in Fig. 2;gray bars summarize the results of other paleoclimate studies fromthroughout the tropical Pacific. Superscripts reference the studiesthat support the climatic and oceanographic changes indicated in thebars: (1) Sandweiss et al. (2001); (2) Rein et al. (2005); (3) Zheng etal. (2008); (4) Koutavas and Joanides (2012); (5) Cobb et al. (2013);(6) Carré et al. (2014); (7) Emile-Geay et al. (2016); (8) Leonard etal. (2016b); (9) Thompson et al. (2017); (10) Toth et al. (2015a);(11) McGregor et al. (2013); (12) Toth et al. (2015b); (13) Conroyet al. (2008); (14) Corrège et al. (2000); (15) Haug et al. (2001);(16) Moy et al. (2002).

Whereas the period from 5 to 3 ka may have been a time oflow ENSO variability on average (Emile-Geay et al., 2016),the time just after 4.2 ka was likely characterized by high cli-matic volatility in the tropical Pacific (Fig. 3). At least onerecord, from Vanuatu in the western Pacific, indicates that amultidecadal period of enhanced ENSO variability followedjust 500 years after the putative low in ENSO activity at∼ 4.2 ka (Corrège et al., 2000); however, we note that thereis less evidence for elevated ENSO variability in a contem-poraneous record from nearby New Caledonia (Emile-Geayet al., 2016). Our data from Pacific Panama suggest that theperiod after 4.2 ka would have been followed by a transitionto La Niña-like conditions by 3.9 ka (Toth et al., 2015a, b),and another recent study suggests that La Niña events werealso more frequent from 3.5–3 ka (Thompson et al., 2017).A record of El Niño-related flooding from Peru implies thatthe strongest El Niño events of the Holocene occurred 4–2 ka (Rein et al., 2005), and a series of records from the sameregion suggest a possible peak in the frequency of El Niñoevents around 3 ka (Sandweiss et al., 2001; Moy et al., 2002).We note, however, that recent reanalysis has called the lat-ter conclusion into question (Emile-Geay and Tingley, 2016).The beginning of the late Holocene was also characterized by

a period of high variability in the position of the Intertropi-cal Convergence Zone (ITCZ), supporting the idea that re-gional climatic variability was high at the time (Haug et al.,2001). Taken together, these studies support the conclusionthat ENSO variability was enhanced after 4.2 ka (Conroy etal., 2008; Koutavas and Joanides, 2012; Carré et al., 2014).

Stronger and/or more frequent El Niño and La Niña eventswould have acted to suppress reef development after 4.2 ka(Toth et al., 2012). ENSO variability continued to increaseafter the end of the hiatus in reef development (Conroy et al.,2008; Cobb et al., 2013; Thompson et al., 2017); however,major changes in the mode of ENSO coincided with its ter-mination (Fig. 3). First, the variability in the mean positionof the ITCZ had declined by ∼ 2.5 ka (Haug et al., 2001).Second, although ENSO variability and the absolute num-ber of El Niño events may have increased beginning 2 ka(Sandweiss et al., 2001; McGregor and Gagan, 2004; Con-roy et al., 2008; Thompson et al., 2017), El Niño events werelikely stronger (Rein et al., 2005) and La Niña events mayhave been more frequent (Thompson et al., 2017) during thehiatus. Our records suggest that upwelling was more moder-ate after the hiatus (Fig. 2a), which would indicate that therewere fewer or less extreme La Niña events. Furthermore, be-cause the climate remained relatively cool after the hiatus(Fig. 2b), it is likely that the influence of El Niño events hadalso decreased. We hypothesize that the waning influence ofLa Niña and reduction in El Niño strength permitted accre-tion to resume between ∼ 1.8 and 1.5 ka (Toth et al., 2012,2015a).

We conclude that ENSO was likely the primary driver ofthe collapse of coral reefs in Pacific Panama and, perhaps,other locations around the tropical Pacific at ∼ 4.2 ka. Thefact that the shutdown in reef accretion occurred in both theGulf of Panama and the Gulf of Chiriquí excludes upwellingand outbreaks of Acanthaster as drivers: upwelling is weakor absent in the Gulf of Chiriquí, and Acanthaster is absentfrom the Gulf of Panama. Other possible factors, includingchanges in relative sea level, tectonics, and bioerosion, can-not explain the observed patterns either (Toth et al., 2012,2015a). The potential role of changing ENSO variability inecological changes elsewhere at this time is less clear, butthere is some evidence to support the idea: increased ENSO-like variability in the climate of eastern China is temporallycorrelated with an inferred increase in coral bleaching around4.2 ka (Li et al., 2018; Xu et al., 2018). Furthermore, even ifenhanced ENSO variability triggered reef shutdown in theTEP at 4.2 ka, that does not mean ENSO-related changesnecessarily accounted for the next 2500 years of suppressedreef development; Fig. 3 implies a concatenation of condi-tions that might or might not have been dependent or semi-independent. Some evidence does in fact point to climaticshifts around the time of the 4.2 ka event that persisted formillennia (Selvaraj et al., 2008).

There is still considerable disagreement about timing andmagnitude of the putative shifts in ENSO variability during

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112 L. T. Toth and R. B. Aronson: The 4.2 ka event, ENSO, and coral reef development

the middle to late Holocene (Emile-Geay et al., 2016). Oneproblem in linking ENSO as a driver or response to the 4.2 kaevent is the asymmetry in the ENSO signal between the east-ern and central Pacific that can be observed from both re-cent El Niño events (Fig. 4; Takahashi et al., 2011) and theHolocene record (Carré et al., 2014; Karamperidou et al.,2015). Indeed, there appear to be two distinct “flavors” ofENSO based on the location of the strongest thermal anoma-lies – eastern Pacific and central Pacific events – and the dom-inance of these ENSO modes may have varied over millen-nial timescales (Karamperidou et al., 2015). Another prob-lem is the large temporal uncertainties associated with manyof the existing paleoclimate records of ENSO. Currently, it isnot possible to clearly resolve the centennial-scale shifts inENSO variability around 4.2 ka. Reducing the temporal un-certainties in paleo-ENSO records at 4.2 ka and the dominantflavor of ENSO at this time, by targeting ENSO proxies fromthroughout the tropical Pacific, will be a critical step to de-termining whether shifts in ENSO variability can be linkedto the 4.2 ka event on a geographic scale and the subsequent2.5 millennia of impacts on reefs throughout the tropical Pa-cific.

4 Regional- to global-scale impacts of ENSO andthe 4.2 ka event

ENSO is the most prominent interannual driver of global-scale climate variability. Although it originates in the trop-ical Pacific, it impacts regional climates around the world(McPhaden et al., 2006). The global impacts of ENSO areprimarily a result of the influence of ENSO-based sea-surfacetemperature anomalies on other drivers of climate variabil-ity such as the ITCZ, the Asian monsoon, and the North At-lantic Oscillation (NAO). For example, the warm (cool) trop-ical Pacific sea-surface temperatures associated with El Niño(La Niña) generally drive a southerly (northerly) shift in themean position of the ITCZ, which results in changes in pre-cipitation from the tropical Pacific to the neotropics, to Africa(Haug et al., 2001; Chaing et al., 2002; Sachs et al., 2009).Similarly, the Asian monsoon is generally suppressed duringEl Niño events and enhanced during La Niña events (Liu etal., 2000; Wang et al., 2003). Although the relationship be-tween ENSO and the NAO, which has a strong influence onEuropean climates, is less straightforward, the NAO is typ-ically negative (positive) during El Niño (La Niña) events(Rodríguez-Foncesca et al., 2016).

The global impacts of El Niño and La Niña events cansignificantly vary from event to event; however, there arestriking spatial correlations between the global signature ofENSO (summarized in Davey et al., 2013) and that of the4.2 ka event. In the majority of the records to date, the 4.2 kaevent has been characterized by changes in terrestrial hy-droclimate (Weiss, 2016). Whereas the event was character-ized by widespread aridification in southern Europe, the Mid-

dle East, northern Africa, midcontinental North America,and parts of eastern and southwest Asia (Weiss et al., 1993;Staubwasser et al., 2003; Marchant and Hooghiemstra, 2004;Booth et al., 2005; Xiao et al., 2018), other locations, suchas western South America, experienced marked increases inprecipitation (Marchant and Hooghiemstra, 2004). With theexception of Europe, which is weakly teleconnected withENSO, the mean shifts in precipitation that occurred dur-ing the 4.2 ka event are generally consistent with those ob-served during a typical El Niño event (Davey et al., 2013).The changes in hydroclimate during typical El Niño eventseven replicate the zonal gradient in precipitation observedacross northern South America at ∼ 4.2 ka (Marchant andHooghiemstra, 2004; McPhaden et al., 2006; Davey et al.,2013).

Like many records of ENSO variability from the tropi-cal Pacific, our records from Pacific Panama do not displaya sudden change in El Niño activity at 4.2 ka (Toth et al.,2015a; Emile-Geay et al., 2016); however, most reconstruc-tions of Holocene ENSO suggest that there was a transitionto increasing ENSO variability around this time (Conroy etal., 2008; Koutavas and Joanides, 2012; Cobb et al., 2013;Carré et al., 2014). Whereas the 4.2 ka event manifested asan abrupt short-lived climatic event in many locations, thelarger-scale climatic and ecosystem responses to the eventmay have been more protracted. Increased ENSO variabil-ity could in this sense be a response to the abrupt climatechanges associated with the 4.2 ka event. In Pacific Panama,the onset of reef collapse just after 4.2 ka was associatedwith a transition to a cooler, La Niña-like climate (Toth etal., 2015; see also Cabarcos et al., 2014), which triggered aregime shift that lasted for millennia. Similarly, there is ev-idence that the onset of aridification in the mid-continentalUnited States was followed by glacial advances in westernCanada, from 4.2 to 3.8 ka, associated with a cooler, wet-ter climate (Menounos et al., 2008; Mayewksi et al., 2004)that is characteristic of La Niña conditions in that region(Davey et al., 2013). Whereas a number of records indicate aweaker East Asian monsoon around 4.2 ka, indicative of anEl Niño-like climate (Staubwasser et al., 2003), there is alsoevidence that some regions of southern Asia became wetter,or more La Niña-like, after 4.2 ka (reviewed in Wu and Liu,2004). Thus, although many of the abrupt changes in terres-trial hydroclimate during the 4.2 ka event resemble those ob-served during El Niño events, the ecosystem changes follow-ing the 4.2 ka event could also reflect responses to secondarychanges in regional climatic systems.

We are not arguing that ENSO was necessarily an ulti-mate cause of the 4.2 ka event; however, the shift in ENSOvariability after 4.2 ka and the similarity between the globalimpacts of ENSO and the global footprint of the 4.2 kaevent suggest a potential connection. The climatic forcingthat produced the changes in ENSO variability during themiddle Holocene are still being debated, but it is generallythought that changes in ENSO during the Holocene have

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L. T. Toth and R. B. Aronson: The 4.2 ka event, ENSO, and coral reef development 113

Figure 4. Thermal anomalies associated with two of the strongest El Niño events (1997–1998 and 2015–2016) and La Niña events (1988–1989 and 2010–2011) in recent history. Black points on the maps indicate the locations of coral reefs. The imagery was created with datafrom NOAA’s Smith and Reynolds Extended Reconstructed Sea Surface Temperature, Level 4 monthly dataset (V5) using NASA’s PO.DAACLAS V8.6.1 visualization tool (https://podaac-tools.jpl.nasa.gov/las/, last access: 1 July 2019).

Table 1. Summary of records of perturbations to coral reef ecosystems around the time of the 4.2 ka event. Reported radiocarbon ages werecalibrated with the marine radiocarbon curve and regional marine reservoir age corrections, using CALIB software (v.7.0.2; Stuiver et al.,2018).

Type of impact Location Time period (kyr BP) Reference

Hiatus in reef Pacific Panamadevelopment Contadora 4.3–1.5 Toth et al. (2012)

Iguana 4.2–1.6 Toth et al. (2012)Canales de Tierra 4.0–1.8 Toth et al. (2012)

Costa RicaGolfo Dulce 4.6–2.0 Cortés et al. (1994)

Great Barrier ReefMoreton Bay 4.2–2.0 Lybolt et al. (2011)

JapanKodakara Island 4.4–4.0 Hamanaka et al. (2012)

Shutdown of reef development HawaiiKailua, O’ahu 5.0–present Rooney et al. (2004)Punalu’u, O’ahu 4.9–present Rooney et al. (2004)Hale o Lono, Moloka’i 4.8–present Rooney et al. (2004)Mana, Kaua’i 4.5–0 Rooney et al. (2004)

Coral mortality ChinaSouth China Sea 4.2–3.8 Xu et al. (2018)

Decline in coral δ13C Pacific PanamaContadora 3.9–3.6 Toth et al. (2015)

ChinaSouth China Sea 3.8 Xu et al. (2018)

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114 L. T. Toth and R. B. Aronson: The 4.2 ka event, ENSO, and coral reef development

been driven by gradual changes in the climate system asso-ciated with changing insolation and/or feedbacks with an-nual climate cycles (McPhaden et al., 2006; McGregor etal., 2013; Emile-Geay et al., 2016;). For example, several re-searchers (Koutavas et al., 2006; McGregor et al., 2013) havesuggested that the weaker ENSO variability observed in thecentral Pacific at ∼ 4.3 ka was related to the more northerlyITCZ at this time, which would have also enhanced east-ern Pacific upwelling, produced cooler sea temperatures inthe TEP, and driven a stronger zonal gradient in sea-surfacetemperature that suppressed ENSO (Clement et al., 2000).In the absence of a clear high-latitude driver for the 4.2 kaevent, these changes in the tropical ocean must be consid-ered as potential contributors to the 4.2 ka event (Marchantand Hooghiemstra, 2004).

5 Prospectus

Because the global teleconnections of El Niño and La Niñacan significantly vary from event to event (Fig. 4; McPhadenet al., 2006), ecosystems of the tropical and subtropical Pa-cific are likely to yield the best evidence for evaluating thepotential role of ENSO in the 4.2 ka event. Corals and coralreef communities that were living under chronically stressedconditions should have been particularly vulnerable to in-creased ENSO variability after 4.2 ka. If the hiatus was trig-gered by La Niña-like conditions (Fig. 4), the 4.2 ka eventshould be most strongly manifested in reef records from ar-eas of upwelling and in seasonally cooler subtropical en-vironments (compared with less-seasonal tropical environ-ments). Nearshore habitats, which are more influenced bythe thermal swings and high sedimentation rates associatedwith terrigenous input, should be more vulnerable comparedwith reefs exposed to clearer oceanic waters. Reefs off thePacific coast of Panama and Costa Rica, which are exposed toa strong terrigenous influence and (in some areas) strong up-welling, display the hiatus (Table 1; Cortés et al., 1994; Tothet al., 2012). Latitudinally marginal lagoonal habitats else-where in the Pacific should also manifest the hiatus (see Cruzet al., 2018). Preliminary evidence corroborates that hypoth-esis in the western Pacific in turbid lagoonal habitats south ofthe Great Barrier Reef, in southern China, and in Japan (Ta-ble 1; Lybolt et al., 2011; Hamanaka et al., 2012; Yamano etal., 2012; Xu et al., 2018); however, falls or stillstands in rela-tive sea level in the western Pacific after the middle Holocenelikely also contributed to stalled vertical reef growth dur-ing the late Holocene in many locations (e.g., Dechnik etal., 2018), making it more difficult to discern the impacts ofclimatic variability and also potentially introducing variabil-ity into the start times of hiatuses in reef accretion. Otherpossible causes of variations in the start times listed in Ta-ble 1 include the artifactual absence of coral material of par-ticular ages in particular samples and the uncertainty asso-ciated with the dating techniques. Whether La Niña-driven

reef shutdown occurred broadly on subtropical reefs of thecentral Pacific remains a topic of active research, but thereis some evidence that increased wave energy tied to ENSOsuppressed vertical reef accretion in Hawaii around the timeof the 4.2 ka event (Table 1; Grossman and Fletcher, 2004;Rooney et al., 2004).

By contrast, the impacts of increased El Niño intensity thatfollowed the putative increase in La Niña activity (Figs. 3, 4)might have been attenuated in lagoonal and nearshore habi-tats, where turbidity associated with terrigenous runoff fromincreased precipitation (in many locations, though not inPanama) would have decreased light penetration and therebyoffered a measure of protection from bleaching (Perry etal., 2012; Cacciapaglia and van Woesik, 2015). Above somethreshold amplitude, however, the ENSO swings would haveoverwhelmed the capacity of even turbid-water reefs to re-sist bleaching (e.g., Aronson et al., 2002). Furthermore, ElNiño conditions are accompanied by increased precipitationin most places, with potentially negative impacts on corals.

Across the equatorial Pacific, contemporary corals andcoral reefs at longitudes that have experienced higher tem-peratures over the previous century have proven more resis-tant to bleaching events. The eastern and central Pacific, in-cluding tropical as well as subtropical areas, have thus beenmore vulnerable for historical reasons than the western Pa-cific (Thompson and van Woesik, 2009). If the short-termgeographic patterns of ENSO history are indicative of pat-terns earlier in the Holocene, then on the basis of El Niño-type impacts we would expect the 4.2 ka event to have man-ifested strongly in the eastern and central Pacific, and lessso in the western Pacific. This hypothesis could be tested bycoring Holocene reef frameworks across the Pacific near theEquator. The physiographic marginality of individual reefs,resulting from additional sources of environmental variabil-ity (such as upwelling) and their particular ecologies (in-cluding their species diversity), would have decreased theirresilience and thereby increased the likelihood that El Niñodisturbances would have left a signature in their fossil record.

Cobb et al. (2013) documented low ENSO variability at4.2 ka at Kiritimati in the Line Islands of the central Pacific.That result potentially falsifies our hypothesis of an event at4.2 ka being strongly manifested in reef frameworks of thecentral Pacific, as well as the idea that contemporary geo-graphic patterns of ENSO variability reflect patterns that pre-vailed 4.2 ka. There is some evidence that the eastern Pacificmode of ENSO was dominant after ∼ 4.5 ka (Carré et al.,2014), which may explain the muted response to the 4.2 kaevent in the central Pacific. Parsing the spatially variable im-pacts of ENSO and other drivers, such as relative sea level,upwelling, climatic effects other than ENSO, and local phys-iography and hydrography, to explain patterns of reef devel-opment will be a significant challenge going forward (forthe Great Barrier Reef: Dechnik et al., 2015; Leonard et al.,2016a, b; Ryan et al., 2018; Webster et al., 2018; see Toth etal., 2016, for the eastern Pacific).

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L. T. Toth and R. B. Aronson: The 4.2 ka event, ENSO, and coral reef development 115

The preponderance of research to date on the causes andconsequences of the 4.2 ka event has focused on terrestrialenvironments and cultural impacts. In this paper we haveshown that there were significant perturbations to coral reefenvironments in Pacific Panama and throughout the tropicalPacific around 4.2 ka. Without a clear high-latitude driver forthe 4.2 ka event, it stands to reason that the climatic changesaround this time could have originated in the tropical oceansand may have been related to contemporaneous changes inENSO variability. Coral reef archives could provide the keyto discerning the role of ENSO in the 4.2 ka event.

A strong role for ENSO in the 4.2 ka event would have im-portant implications for living coral populations and futurereef accretion in the TEP. Contemporary climate change willlikely increase ENSO variability in the eastern Pacific (Caiet al., 2018). A robust connection between ENSO variabilityand reef development could, therefore, portend another shut-down of eastern Pacific reefs.

Data availability. Some of the data reviewed in this paper areavailable through references cited in the figure captions.

Author contributions. LTT and RBA contributed equally to theconception and writing of this manuscript.

Competing interests. The authors declare that they have no con-flict of interest.

Special issue statement. This article is part of the special is-sue “The 4.2 ka BP climatic event”. It is a result of “The 4.2 ka BPEvent: An International Workshop”, Pisa, Italy, 10–12 January2018.

Acknowledgements. This paper grew out of an internationalworkshop, “The 4.2 ka BP Event”, which was organized byGiovanni Zanchetta, Harvey Weiss, and Monica Bini and heldin the Dipartimento di Scienzi della Terra at the University ofPisa in January 2018. We thank the organizers for the opportunityto participate in the workshop. Raymond Bradley, Mark Bush,Evan Edinger, Nicholas Graham, William Precht, Julie Richey,Robert van Woesik, Harvey Weiss, and two anonymous reviewersprovided valuable advice and discussion. Our research wassupported by grant OCE-1535007 from the US National ScienceFoundation. Toth’s research is also funded by the Coastal andMarine Hazards and Resources Program of the United StatesGeological Survey. This is contribution number 203 from theInstitute for Global Ecology at the Florida Institute of Technology.Any use of trade, firm, or product names is for descriptive pur-poses only and does not imply endorsement by the US Government.

Edited by: Harvey WeissReviewed by: Evan Edinger and two anonymous referees

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